Solar power stations that use a sensible heat storage medium, typically a molten salt, have been proposed to store captured solar energy. The sensible heat storage medium can then be used for steam generation to drive conventional steam turbine generator systems.
It is advantageous to store energy at higher temperatures because it minimises the mass of the storage medium for a sensible energy storage medium, and allows energy recovery from storage at higher temperature, which leads to higher conversion efficiencies in the power generation cycle.
Energy is transferred from the solar collector to the sensible energy storage medium by a heat transfer fluid. Synthetic oils have been proposed as a heat transfer fluid but these are typically only stable to about 400° C., limiting the maximum temperature of the sensible heat storage medium. Typically oil is used as the heat transfer medium for solar trough plants, including plants with molten salt thermal storage. However solar troughs are limited in the upper temperature they can operate at with acceptable efficiency, as they track the sun on a single axis and therefore have a lower solar concentration ratio than two axis tracking solar collectors, such as dishes. The use of oil for a solar trough plant does restrict the upper temperature they might otherwise operate, from say 450° C. to around 400° C.
Solar dish and tower technologies have the capacity to reach far higher temperatures, in excess of 600° C., which has the advantages previously mentioned of minimising the mass of the storage medium and facilitating higher conversion efficiencies in the power generation cycle. Unfortunately, there is no fluid that remains in liquid state at both such high temperature and at ambient temperature (with the exception of Sodium-Potassium alloy (NaK) which is considered too dangerous for a solar pipe network as it is highly reactive with water and may explode when in contact with water or air).
Typically solar tower technologies, which have a fixed solar receiver, directly heat molten salt at the focal point which is located in close proximity to the storage vessels. Reticulation of the salt, which acts as both the energy storage medium and heat transfer fluid, is relatively simple, and gravity drain back ensures the salt is emptied from the pipework during the night or extended cloudy periods and hence cannot freeze in the pipes or receiver. For solar dish technologies, the direct heating solution is problematic as each dish has its own focal point and dishes may be distributed over a very large area. Emptying the pipes may be impractical or unfeasible. Salt mixtures used for thermal storage are solid at ambient temperatures, hence a distributed solar field using salt as both the heat transfer fluid and the storage medium would be required to be heated at all times, day and night. Any flaw in the heating system would result in freezing of salt in the pipe network.
Water can be used as a heat transfer fluid. When using water as the heat transfer fluid over a high temperature range there is typically a phase change at practical working pressures. As the water is cooled and changes phase from superheated steam to liquid water there is a well known “pinch point” problem with transferring energy from steam to a single sensible heat storage medium. This “pinch point” problem significantly limits the upper temperature of the heat storage even when high temperature steam is available.
For heat to be transferred from the water (whether vapour or liquid) to the storage medium, the water must be at a higher temperature than the storage medium at all points along the heat exchanger. When condensing vapour to liquid the temperature remains the same (at constant pressure) between 100% vapour and 100% liquid. This causes a pinch point at the 100% vapour point which limits the maximum temperature that can be attained in the storage medium. The pinch point problem is illustrated in a temperature-enthalpy diagram in FIG. 1, which shows the enthalpy of water 2 at 165 bar from 300° C. to 600° C. and the enthalpy of a sensible heat storage medium 4 heated by the water. Despite the high inlet steam temperature in this example, a maximum temperature of the storage medium is limited by the ‘pinch’ to about 380° C.
It has been proposed to separate the steam flow into three sections: pure liquid, 2-phase liquid and vapour, and pure vapour. In that method, three separate heat storage mediums are required. There is sensible heat storage for both liquid and vapour phases, and a phase change material (such as an appropriate salt) for the 2-phase region. Whilst this avoids the pinch point problem it would result in a relatively complex energy storage system.
Solar power plants that use solar dishes convert sunlight to electricity at almost twice the efficiency of other technologies (tower or trough) due to the combination of high temperature capability and high optical efficiency. However, power generation with solar dishes when combined with molten salt storage is problematic when using heat transfer fluids in the conventional way for the reasons aforementioned.